“Glasgow isn’t performing," the chief executive officer of New Zealand-based Infratil, Marko Bogoievski, said in an interview. “It’s a difficult asset to see in the portfolio in the long term."

Combined passenger numbers at Prestwick and Infratil’s other UK airport, Manston, tumbled 33 per cent in April through August, predominantly because Ryanair cut flights at its Glasgow hub.

Infratil may use the proceeds from selling Prestwick for investments in other airports or to boost its portfolio of energy assets in Australia and New Zealand, which offer more reliable earnings, Mr Bogoievski said.

Investments with strong cash flows “are easier for investors to get their head around and easier for bankers to bank," he said. “We’ve rebalanced to that sort of asset."

Over the past two years Infratil has also sold a 90 per cent stake in Germany’s Luebeck Airport and its shares in Auckland International Airport.

In March 2010, it joined with the New Zealand Superannuation Fund to buy Royal Dutch Shell’s NZ fuel retailing assets.

But Mr Bogoievski said Infratil has no plans to shed its 66 per cent stake in Wellington Airport, even as investor Utilico Investments calls for a sale. London-based Utilico, which owns about 15 per cent of Infratil, said last month the stake should be sold because the investment is mature.

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“They’re an investor and entitled to their view," Mr Bogoievski said. The company will retain the Wellington stake as earnings are growing and because passenger numbers will probably double by 2030, he said.

Infratil said last week that first-half earnings will likely be in line with full-year guidance as it benefited from higher electricity prices and increased bus demand because of the Rugby World Cup.

The shares fell 0.5 per cent to $NZ1.83 in Wellington trading, extending the stock’s decline this year to 5.7 per cent. The benchmark New Zealand NZX 50 index has gained 1.9 per cent.

Prestwick, Scotland’s busiest cargo airport, and Manston, east of London, handled 560,289 passengers in the five months to the end of August.The Scottish airport is about 50 kilometres south of Glasgow.

Glasgow’s larger airport, Glasgow International, or Edinburgh Airport, Scotland’s busiest, may also go on sale soon as the UK’s competition commission has ordered BAA to shed one of the two facilities. BAA, the owner of London’s Heathrow, is appealing.

Prestwick and Manston made a combined operating loss of $NZ11.3 million ($8.9 million) in the year ended March 31, prompting Infratil to cut their value to $NZ101 million from $NZ138 million a year earlier. Passenger numbers fell 9.4 per cent in the year ended March, while freight volumes tumbled about 20 per cent.